1. Field of the Invention
The present invention relates generally to a spark ignition timing control system for an internal combustion engine, especially for the an internal combustion engine using gasoline as a fuel. More specifically, the invention relates to a spark ignition control system which is effective for suppressing engine knocking and has variable knocking responsive spark advance control characteristics depending on the grade of gasoline used.
2. Description of the Background Art
Recently, a greater variety of gasoline for internal combustion engines has become available. For instance, an unleaded and high octane value gasoline is becoming popular due to low exhaust emission and high engine performance. One advantage of such high octane value gasoline is high knocking suppressing capability to allow spark advance to be maintained at a relatively advanced angle for higher engine performance. Because of this high knocking suppressing capability, the angular range of spark advance for such high octane value gasoline can be shifted toward the advance side in comparison with that of regular octane value gasoline. In other words, as long as the spark advance variation range is held in a range for regular octane value gasoline, spark advance cannot be advanced beyond the advance limit set for regular octane value gasoline, even when high octane value gasoline is used. On the other hand, when the spark advance angular region is set to effectively use high octane value gasoline, a knock suppressing spark advance retarding magnitude, when regular octane value gasoline is used, becomes insufficient and causes substantial engine knocking and engine damage.
In view of this, Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-212673 proposes a variable spark advance variation range including one spark advance control schedule adapted for high octane value gasoline, which will be hereafter referred to as a "high octane value spark advance schedule", and spark advance control schedule adapted for regular octane value gasoline which will be hereafter referred to as a "regular octane value spark advance schedule". The disclosed system further introduces automatic gasoline discrimination capability by initially selecting the regular octane value spark advance schedule. In discrimination stage operation, the spark advance is advanced until unacceptably heavy engine knocking is detected. An angular position at which unacceptably heavy knocking is detected is latched. The spark advance at which heavy engine knocking is monitored will be hereafter referred to as a "knocking spark advance point". Discrimination of gasoline the used is performed on the basis of the latched knocking spark advance point. Namely, because of different knocking suppressive capabilities between the high octane value gasoline and regular octane value gasoline, the knocking spark advance point is different, and discrimination is made by comparing a monitored knocking spark advance point with a criterion which may be set near the possible most advanced spark advance angle in a regular octane value spark advance schedule.
Though such a system works successfully in discriminating the kind of gasoline and in selecting one of the high octane value spark advance schedule and the regular octane value spark advance schedule, it necessarily causes a sudden change of the spark advance control schedule when the gasoline to be used is varied. This may cause a substantial change in engine output performance and give unacceptably an uncomfortable engine drive feeling to the driver. For instance, when the kind of gasoline is switched from high octane value gasoline to regular octane value gasoline, the spark advance control schedule is switched from the high octane value spark advance schedule to the regular octane value spark advance schedule to retard the variation range of spark advance. This causes a lowering of the engine output performance and degradation of the vehicular acceleration characteristics.